The present invention relates to a motor drive particularly for submersed electric pumps.
As is known, submersed electric pumps are constituted by a multistage pumping unit associated with a submersed electric motor.
Submersed electric motors are substantially constituted by a cylindrical casing, which is closed hermetically by a head and a bottom and is adapted to contain a stator within which a rotor connected to the motor shaft rotates.
Very often, the stator and the rotor are immersed in a bath of dielectric cooling liquid that fills the casing completely in order to optimize the cooling of the submersed motor.
Exchange of liquids between the inside and the outside of the casing is prevented by a rotary mechanical seal, which is arranged at the output of the motor shaft. The seal is substantially constituted by a rotating ring, which rotates jointly with the shaft, and by a stationary ring, which is fixed to the casing. The two rings, which move with a relative motion, are mutually n contact by virtue of highly polished faces and the contact force is generally ensured by springs.
The volume of the dielectric cooling liquid varies according to both the operating temperature of the motor itself and the value of the pressure of the environment in which the motor is located. In order to allow variations of the volume of the dielectric cooling liquid, generally an elastic compensation membrane is provided at the bottom of the casing. The membrane is adapted to ensure that the pressure inside the casing is substantially equal to the outside pressure.
However, in some cases, the presence of conventional membranes can be disadvantageous.
It must in fact be noted that submersed electric motors are more and more often associated with power supply and control units, specifically inverters, which ensure a more efficient use of the motors.
In order to compact the dimensions and prevent the generation of overvoltages that might compromise the electric motor, the power supply and control units must be placed as close as possible to the motor. Accordingly, the casing very often includes both the electric motor and the power supply and control unit.
In those cases, a simple conventional membrane on the bottom of the casing is absolutely counterproductive, because by making the pressure inside the casing substantially equal to the outside pressure, all the electronic components are ultimately subjected to the action of the hydrostatic pressure.
However, the hydrostatic pressure can reach very high values and its conditions are highly variable as a function of the depth of the submersed motor with respect to the surface of the liquid in which it is submersed.
In such cases, the power supply and control unit therefore ultimately finds itself in a particularly disadvantageous situation and its electronic components are exposed to severe risk of damage.
It must also be noted that the substantial equivalence between the pressure inside the casing and the outside pressure is scarcely favorable also in the action for contrasting the leaks of dielectric cooling liquid through the rotary mechanical seal.
In the described pressure conditions, the task of ensuring contact between the rotating ring and the stationary one is entrusted completely to the springs that press the sliding fronts of the rings face to face.
Although the thrust of the springs may be sufficient to maintain tightness when the cooling liquid is cold and therefore denser, their mechanical action can turn out to be inadequate when the liquid becomes more fluid due to the heating of the motor, caused for example by the application of a load.